(1) Field of the Invention
The present invention relates to imaging at high temperatures and, more particularly, to an image contrast and enhancement system that enables imaging of a sample in a furnace or the like in a manner providing discernible contrast between the sample and the surroundings thereof.
(2) Description of the Related Art
Stephen""s Law of Radiation provides that radiation from a high temperature source increases by a factor of sixteen with each doubling of the temperature. As a consequence, in a furnace wherein the temperature is about 1500 degrees Centigrade or above, everything radiates as a xe2x80x9cblack body,xe2x80x9d i.e., at an extremely high illumination of light. Accordingly, at these temperatures all items in the furnace look alike, i.e., there is no discernible contrast between the items.
In the process of sintering of a sample (made of, e.g., tungsten, nickel, iron, copper or the like) in a high temperature furnace, sample morphology of considerable interest and thus the contrast between the sample and the sample surroundings is of utmost importance.
One known device for obtaining high contrast images in a very high temperature environment is sold under the name PYROCAM by Control Vision Inc. The device uses a high intensity Xenon flash lamp that produces a five microsecond pulse and employs a bandpass filter to permit only the blue line of Xenon flash lamp to be used in obtaining the images. A digital process is used to provide further image enhancement. One disadvantage of the device is that the device operates off axis, i.e., the incident beam forms a small angle with the received beam. In addition, a relatively large opening (at least three inches in diameter) is required.
According to the invention, an image enhancement system is provided which affords greatly improved image contrast in images of a target in a high temperature environment such as a furnace or the like. Moreover, in contrast to the PYROCAM device, the invention provides an on-axis, monostatic mode of operation that permits the system of the invention to interface with a much smaller opening. Further, the overall device can be made quite small and compact.
In accordance with the invention, there is provided an image enhancement system for providing image contrast in an image of target and its surroundings in a high temperature environment, the system comprising: a light source for producing a light beam; circular polarizing means for circularly polarizing said light beam to produce a circularly polarized light beam directed along an optical path toward said target so as to be reflected by the target and its surroundings; a first linear polarizer disposed in the optical path between said circular polarizing means and the target through which said circularly polarized light beam passes in a first direction which traveling to the target and through which the circularly polarized light beam passes in a second, opposite direction, along with thermal radiation from the target and its surroundings, after being reflected and reversed in polarization by the target and its surroundings; and a second linear polarizer disposed in the optical path downstream of the circular polarizing means and providing polarization of light received thereby which is orthogonal to the polarization provided by said first polarizer, the circular polarizing means converting the reflected and polarization reversed, circularly polarized light received from said first polarizer into light of the same polarization as that provided by the second polarizer so that the second polarizer passes the light received from said circular polarizing means while blocking said thermal radiation so as to provide a high contrast image of the target.
Preferably, the circularly polarizing means comprises a quarter wave retardation plate. The system preferably further comprises a beam splitter disposed in the common optical path for receiving the light beam from the light source, for directing the received light beam along the light path toward the target and for passing said reflected light beam to the second polarizer. Advantageously, the quarter wave retardation plate is mounted on a target-facing surface of the beam splitter. Preferably, the beam splitter comprises a polarizing cubic beam splitter.
In a preferred implementation, a band pass filter is disposed along the optical path downstream of the circular polarizing means which has a band pass filter characteristic matching the frequency of the light beam produced by the light source.
Advantageously, a charge coupled device camera is positioned along the optical path for capturing the high contrast image.
Preferably, a field aperture is positioned along the optical path between the circular polarizing means and the second polarizer. Advantageously, a lens is positioned between the circular polarizing means and the field aperture along the optical path.
In an advantageous implementation, the light source comprises a source of vertically polarized light, and the first polarizer comprises a vertical polarizer and the second polarizer comprises a horizontal polarizer.
In a preferred embodiment, the light source comprises a laser light source.
In accordance with a further aspect of the invention, there is provided an optical image enhancement system for providing image contrast in an image of a target in high temperature surroundings, the optical system comprising: a source of vertically polarized light; a beam splitter for receiving said light and directing the light toward the target; a retardation plate affixed to a target-facing surface of the beam splitter; a vertical polarizer disposed along a common optical path with said beam splitter between the retardation plate and the target; a horizontal polarizer disposed in the common optical path for receiving light passing through a surface of said beam splitter opposed to said target-facing surface; an image detector disposed at one end of the common optical path; and a band pass filter disposed in the common optical path between the horizontal polarizer and the image detector and having a band pass filter characteristic matching the frequency of the vertically polarized light source.
Advantageously, the beam splitter comprises a cubic polarizing beam splitter. Preferably, the system further comprises a field aperture disposed in the common optical path between the beam splitter and the second polarizer. Advantageously, a lens is positioned between the beam splitter and said field aperture.
The image detector preferably comprises a charge coupled device camera.
The light source preferably comprises a laser.
Further features and advantages of the present invention will be set forth in, or apparent from, the detailed description of preferred embodiments thereof which follows.